Infestation of the body by lice is an age old problem. Reference to these pests can be found throughout documented history. Lice have been responsible for the spread of typhus, causing decimation of many armies and navies of the military powers of the 15th, 16th, 17th and 18th centuries. Lice are still considered as disease vectors and present serious health problems throughout the world. Not only do lice carry a wide variety of bacteria on their exterior surfaces, but their fecal matter transmits disease when it enters the puncture wounds lice inflict during feeding.
The human lice genus includes pubic lice, body lice, and head lice. Although related, each of them have specific characteristics with regard to habitat and feeding. The present invention is most useful in treatment of head lice. Head lice are small hard-shelled ectoparasites which cling to hair follicles while feeding, mating and laying eggs. The louse must remain on the head as it will die within a short period of time when removed. Head lice proliferate at an incredible rate. A louse is ready to mate and reproduce within 10 hours after hatching. Under ideal conditions, a female louse may produce up to 300 eggs in its lifetime. Ideal conditions include an adequate food supply, environmental temperatures from about 28.degree. C .to about 32.degree. C., and relative humidity from about 70% to about 90%. Poor hygienic and grooming habits are also known to contribute significantly to the spread of lice. Thus, lice infestations are most serious in tropical areas where the inhabitants have both substandard hygienic facilities and practices.
The louse's hard keratinous shell serves as protection from external elements. Lice eggs (or ova) are similarly protected by a chitinous sheath surrounding the eggs and attached to the hair follicles. Although the lice may be affected by the use of an insecticide, often the eggs remain resistant to attack. Thus, the optimum treatment should kill the adult lice and interrupt the gestation of the eggs.
Biologically active agents for the control of lice are well known in the art. Lindane (gamma-benzene hexachloride), synergized natural pyrethrins, and synthetic derived compounds known as pyrethroids have all been used as pediculicides in lice treatment compositions. However, since lindane has a poor safety profile and lice have developed a significant degree of resistance to it, natural pyrethrins and synthetic pyrethroids are routinely chosen for use in pediculicide and ovicide compositions.
Natural pyrethrins are made from extracts of naturally insecticidal chrysanthemum flowers and have been used since the early 1930's. European Patent Application 191,236 published Aug. 20, 1986; European Patent Application 262,885, published Apr. 6, 1988; and British Patent Specification 1,593,601, published Jul. 22, 1981, all disclose the use of natural pyrethrins for treating lice. U.S. Pat. No. 4,668,666, Allan, issued May 26, 1987, notes that natural pyrethrins' poor environmental stability is a severe drawback in treatment of lice, as its low residual action, due to this instability, necessitates frequent follow-up treatments.
Synthetic pyrethroids became popular during World War II when chrysanthemum flowers became nearly impossible to get. Besides being cheaper and available, they were also somewhat more stable than the natural product. This long-term stability extended toxicity to future hatching ova. Although generally more effective against lice than natural pediculicides, some of the synthetic actives are more toxic to the subject being treated.
Anti-lice formulations eliminating insecticides are known in the art. Alkanols are known to effectively control head lice infestations. British Patent Specification 1,604,857, published Dec. 16, 1981, and European Application 262,885, Gordon, published Apr. 6, 1988, disclose that in general, alkanols either singularly or as adjuvants, may be toxic to lice. Primary as well as aromatic alkanols are disclosed therein as components of anti-lice compositions, such as sprays and shampoos. Phenyl ethanol is specifically disclosed in the British Patent Specification as a lone toxicant used at a level of 15%. No level is disclosed when it is used as an adjuvant.
Cationic surfactants are known as synergizers for pediculicides; see British Patent Specification 1,593,601, published Jul. 22, 1981. U.S. Pat. No. 4,183,913, Enders et al., issued Jan. 15, 1980, specifically discloses quaternary ammonium salts as synergizers for pediculicides. Quaternary ammonium salts are disclosed as the lone pediculicide for head lice in European Patent Application 191,236, published Aug. 20, 1986.